The present invention relates to an image forming method (hereinafter, this method is simply called KNC), by which a latent image which is color-separated, is formed on an image forming body, multi-color toner images are superimposed on the image forming body, and after that, toner images are transferred onto a recording sheet. More specifically, the present invention relates to a color image forming method by which color reproduction of an edge, fine lines, or isolated points of an image is increased, and to a color image forming apparatus which is used as a printing apparatus, or a copying apparatus, in which this method is adopted.
The KNC process is a process in which multi-color toner images are superimposed on an image forming body by repeating a charging process, an image exposure process, and a reversal development process, and after that, toner images are transferred onto a recording sheet. In the development process, a DC and an AC bias voltage are applied onto each developing sleeve, and the non-contact reversal development is carried out on the image forming body. An adhering condition of toner obtained by the KNC process, is not simply determined only by an exposure which is optically modulated according to image density data. In the case where an external exposure system, in which image exposure means are arranged outside the image forming body, and image exposure is carried out from the outside of the image forming body, is adopted, the following phenomena have relation to the toner adhering condition.
The first phenomenon is one that the next toner hardly adheres onto a solid portion of the toner image because of the shielding property by which the toner layer potential or toner hardly transmits light. This is simply called the average slippage by the structure of the preceding image. The second phenomenon is the deformation of an electrostatic latent image (hereinafter, simply called a latent image), which is produced by the structure of the previously formed toner image, that is, the edge effect which occurs at an edge of an isolated point, an isolated dot-line, letters, and a solid portion, or a halo effect which appears as a false contour phenomenon, when colors are superimposed on each other. The cause of the phenomenon is the same as that of the edge effect, and this phenomenon is one specific to the KNC process by the superimposition. Such a slippage by development is simply called a partial slippage by the structure among images. The third phenomenon is the deformation of a latent image generated by the type of images, without depending on the condition under which no toner image is formed yet on the image forming body, or the structure of the previously formed toner images, that is the edge effect phenomenon specific to the electronic photographic method, and is the slippage between image data and the reproduction image, which is hereinafter simply called the slippage by the structure of the image. Sometimes the edge effect or the halo effect reaches 0.5-2 mm, although also depending on the developing method or characteristics of the photoreceptor layer. In order to suppress such phenomena, conventionally, as disclosed in Japanese Patent Publication Open to Public Inspection No. 218991/1994, or the like, the color reproducibility is increased by the following method, in which the pulse width is modulated at the time of image exposure, so that a toner image is formed well-balanced on the lower layer and the upper layer for each unit of a recording dot, and correction of modulation of the exposure beam is conducted on isolated pixels, a solid portion which is continuous pixels, and edge pixels of the solid portions. Concretely, a correction, in which the first color is weak and the second color is strong, is conducted when the recording dots are superimposed.
FIG. 3 is a graph showing the KNC correction by the external exposure system.
The graph shows the characteristics of the photoreceptor of the image forming body, in which the vertical axis of the graph shows the surface potential of the image forming body, and the horizontal axis of the graph shows the exposure amount.
Further, hereinafter, a color, which is formed with a single color toner such as that of yellow, magenta, cyan and black, is named as "the primary color" and a color, which is formed by a composition of two different primary colors, is named as "the secondary color". The following description will be treated with those names.
The curve a is the photosensitive characteristics under the condition that no toner image is carried on the surface of the image forming body, E.sub.1/2 is an exposure amount required to decay the initial charging potential V.sub.0 to the half value, and this is referred to as the half decay exposure. The exposure amount E.sub.c is an exposure amount to obtain the primary color having the maximum image density, and the exposure amount E.sub.a is an exposure amount of the first color to obtain the secondary color having the maximum image density. The exposure amount E.sub.a is smaller than the exposure amount E.sub.c, so that the toner adhering amount of the first color to obtain the secondary color is made almost the same as that of the second and subsequent color.
The curve b is photosensitive characteristics under the condition that the first color toner layer is already carried on the surface of the image forming body in order to obtain the secondary color. The curve b is a curve having a degree of the decay which is less than that of the curve a because of the light shielding property of the toner, and the remaining potential also rises by the previously formed toner layer potential. Accordingly, for example, when the first color image exposure is carried out with the same exposure amount E.sub.a as that of the second color image exposure in order to obtain the secondary color, the latent image potential is not fully lowered as shown in the graph, and is not the same. Thereby, the second color toner image, which has a smaller toner adhering amount and in which color balance is lost, is obtained. In order to correct the loss of the color balance, the second color exposure amount E.sub.b1 to obtain the secondary color is corrected so as to be larger than the first color exposure amount E.sub.a to obtain the secondary color. The exposure amount E.sub.b1 is approximately twice E.sub.a or more, and is a large exposure amount which is approximately the same as E.sub.c, or more. Thereby, the first color toner layer adhering amount to obtain the secondary color having the maximum density, is made the same as the toner layer adhering amount of the second and subsequent color. This is the basic principle of the KNC correction by the external exposure system, and because the correction amount is large, this is a reason in which the stability of the color image is difficult.
The KNC process, in which the KNC correction is added by the external exposure system, will be described below.
FIG. 4 is diagrams showing the surface potential on the image forming body in the KNC process of the external exposure system.
FIG. 4(a) is a diagram showing the initial charge, and it can be seen that the surface potential of the image forming body is uniformly set to the charging potential. FIG. 4(b) is a diagram showing the exposure process to form the first color latent image, in which E.sub.c shows an exposure amount to obtain the primary color having the maximum image density, and E.sub.a shows the first color exposure amount to obtain the secondary color having the maximum image density. Because E.sub.a is smaller than E.sub.c, it can be seen that the latent image potential is higher.
FIG. 4(c) is a diagram showing conditions after the first color developing process, and it can be seen that toner adheres corresponding to the potential difference between the latent image potential and developing bias voltage.
FIG. 4(d) is a diagram showing the surface potential of the image forming body after the second charging process. It can be seen that the charging potential is uniform without depending on the existence of toner.
FIG. 4(e) is a diagram showing the exposure process to form the second color latent image. E.sub.c shows an exposure amount to obtain the primary color having the maximum density, and E.sub.b1 shows the second color exposure amount to obtain the secondary color having the maximum image density. When E.sub.b1 is the same as E.sub.c or larger than E.sub.c, the influence of the first color toner layer potential, or the light shielding property of toner is corrected, and the second color latent image potential is the same as the first color toner latent image potential.
FIG. 4(f) is a diagram showing conditions after the second color developing process, and toner adheres corresponding to the voltage difference between the latent image potential and the developing bias voltage. That is, it can be seen that the adhering amount of the first color toner layer to obtain the secondary color and that of the toner layer after the second and subsequent color are corrected by the exposure amount so that these adhering amounts are the same.
Further, the same exposure amount correction is also conducted on the primary color and secondary color ranges, which have intermediate image density, ranging from the low to intermediate density, according to the potential characteristics shown in FIGS. 3 and 4.
However, when the KNC process, in which image exposure is conducted from the inside, is carried out, there is no influence of light shielding of the previous toner image, or spread of the beam diameter, caused by light scattering by the toner image, as compared to the KNC process in which the external exposure system is adopted. Concretely, the image exposure is conducted without being influenced by the previous toner image on the above-described points, however, on the other hand, the remaining potential is increased by the toner layer potential. Although the first phenomenon or the second phenomenon is decreased because no light absorption or light scattering occurs, the KNC correction by the conventional external exposure system can not simply be applied because the toner layer exists, and therefore, the correction specific to the internal exposure system is necessary.
Furthermore, proposals in the conventional external exposure system are for binary image data, and are limited to the two color superimposition. The proposals highly regard adjoining pixel information of the image, and can not increase the quality of color reproduction of the edge, fine line, and isolated point, including the case of multi-value image data. Further, the proposals do not meet the case of the full-color image in which 3-4 colors are superimposed. This means the following: wide range correction is essential because the edge effect range is extended to about 1 mm; it is necessary that the correction level is highly accurate for multi-value color image data, and the conventional correction by adjoining pixel information is not adequate; and thereby, the correction is necessary which meets the structure or extension of the image.